Cathode heater assembly for use in strong d.c. magnetic fields



M. F. LISCIO CATHODE HEATER ASSEMBLY FOR USE IN STRONG D.C. MAGNETICFIELDS 2 Sheets-Sheet 1 Filed Sept. 6, 1962 INVENTOR.

MAURlCE F. LISCIO 1% i f/%;

ATTORNEY ec. 14, 1965 M. F. LISCIO 3,223,876

CATHODE HEATER ASSEMBLY FOR USE IN STRONG D.C. MAGNETIC FIELDS FiledSept. 6, 1962 2 Sheets-Sheet 2 INVENTOR. MAURICE F. LISCIO ATTORNEYUnited States Patent M Jersey Filed Sept. 6, 1962, Ser. No. 221,796 4Claims. (Cl. 313-276) The present invention relates in general toelectron discharge devices of the crossed electric and magnetic fieldtype and more specifically to a reverse magnetron useful for generatinghigh power microwave energy at extremely high frequencies such asrequired in high power, high resolution radars.

A reverse magnetron tube typically comprises a circular electric modecavity or circular electric mode wave propagating structure surroundedby a circumferal array of outwardly directed vane or cavity resonatorscoupled to the excited circular electric structure via a circular arrayof axial slots communicating with alternate anode resonators. The arrayof anode resonators are surrounded by a magnetron interaction regionformed by an annular cathode emitter emitting radially inwardly into theanode, in the presence of a strong axial magnetic field. Rotating spokesof electron space charge interact with the 71' mode fields of the anoderesonators to excite the circular electric mode in the circular electricmode cavity. Since the stored energy of the circular electric modecavity is much higher than that of the vane resonator circuit the anodevane resonator system is locked in the 1r mode to the circular electriccavity mode thereby stabilizing the magnetron. The reverse magnetronstructure may be used as an oscillator or as an amplifier and microwaveenergy is extracted from the circular electric mode wave propagatingstructure of cavity and fed to a suitable load.

Heretofore a reverse magnetron of the above described type has beenbuilt operating at approximately 35 gigacycles and generating a peakpower of approximately 150 kilowatts with an average RF. power of 75watts. When an attempt is made to achieve long life and reliability itis found that the prior art design of the heater and heater support wasone of the major factors affecting life.

It was found that the prior art ceramic coated helical cathode A.C.heater element experienced motoring in the strond D.C. axial magneticfringe fields causing the ceramic heater insulating coating to chafe offthe filament at elevated temperatures producing shorting of the heaterfilament thereby reducing tube life to something in the order of fiveminutes.

The reverse magnetron tube of the present invention solves theaforementioned ditficulties associated with the prior art tube andprovides a 32-35 gigacycle magnetron having a peak power output in theorder of 290 kilowatts with average power output of approximately 50watts while yielding overall efficiencies of approximately 30% and aturnable bandwidth of 12%. This tube represents more than an order ofmagnitude increase in peak power output with approximately double thepreviously obtained eificiency while having a long operating life inexcess of 2,400 hours.

The principal object of the present invention is to provide an improvedhigh power reverse magnetron tube yielding substantially enhancedoperating life.

One feature of the present invention is the provision of an insulatingsupport structure of a helical A.C. filamentary cathode heater whereinthe helical A.C. heater element is tangentially supported at spacedpoints about its periphery via refractory longitudinally directedmembers, whereby relative motion between the heater helix 3,223,876Patented Dec. 14, 1965 and the cathode structure is permitted withoutshortening the AC. filament to the cathode structure.

Other features and advantages of the present invention will becomeapparent upon a perusal of the specification taken in connection withthe accompanying drawings wherein:

FIG. 1 is an outside perspective view of the reverse magnetron tube ofthe present invention,

FIG. 2 is an enlarged fragmentary view partly broken away and partly insection of the structure of FIG. 1 taken along the line 22 in thedirection of the arrows,

FIG. 3 is a fragmentary view partly in cross-section and partly brokenaway of the portion of the structure of FIG. 2 taken along the line 33in the direction of the arrows,

FIG. 4 is an enlarged fragmentary cross-sectional view of a portion ofthe structure of FIG. 2 delineated by line 44 and rotated in the counterclockwise direction,

FIG. 5 is an enlarged perspective view of a portion of the structure ofFIG. 4 delineated by line 5-5,

FIG. 6 is an enlarged perspective view of an alternative structure tothat of FIG. 5, and

FIG. 7 is an enlarged cross-sectional view of an alternative structureto that portion of the structure of FIG. 4 delineated by line 7-7.

Referring now to FIGS. 1, 2 and 3, character 1 represents the hollowtubular supporting body of the reverse magnetron, as of copper, to whichother parts are brazed or otherwise suitably fastened to form astructure capable of being evacuated. On opposite sides of the body 1 inaxial alignment there are brazed to the body 1 a tubular outputwaveguide assembly 2 and tuner assembly 3. Cathode lead-in insulatorstructure 4 extends outwardly from the main body of section 1 inquadrature with the axially aligned output waveguide and tunerstructures 2 and 3 respectively.

The term circular electric mode cavity as used herein is defined to meana cavity formed, dimensioned, and excited in such a manner as to supportat its certain pre selected operating frequency a certain circularelectric mode, of the general form TE to the exclusion of other modes. Acircular electric mode cavity typically includes an outer cylindricalside wall and may or may not have an axially directed center conductor.

A circular electric mode cavity 5 is disposed centrally of the anodebody 1 on the axis of the tube. In amplifier embodiments of the presentinvention the circular electric mode cavity 5 is replaced by a circularelectric wave propagating wave structure such as, for example, a hollowcylindrical pipe having an input port as well as an output port. Acircumferal array of outwardly directed vanes 6 surround the circularelectric mode cavity 5 and from an array of anode resonators by thespaces between adjacent vanes 6. Alternate anode resonators areelectromagnetically coupled to the circular electric mode cavity 5 viaan array of axially directed slots 7 communicating through the commonwall between the anode resonators and the circular electric mode cavity5. A magnetron interaction region 8 surrounds the outer tips of thevanes 6 and is defined by the space in between the vanes 6 and asurrounding cathode emitter ring 9.

A strong axial magnetic field of 12,000 to 15,000 gauss for themagnetron interaction region 8 is provided by a magnet 11, onlypartially shown in FIG. 2, enveloping the anode body portion 1 andhaving a re-entrant internal magnetic gap extending in the axialdirection through the magnetron interaction region 8 between themagnetic pole pieces 12 disposed on opposite sides of the anode vanes 6.

Tuning of the tube over its approximate 12% tuning band, centered atapproximately 34 gigacycles, is obtained by means of axial translationof a combined cavity end wall and output coupling plate 13 carried uponthe end of an axially directed and positioned rod 14 which is axiallytranslatable via the intermediary of a captured nut 15 and bellowsassembly 16, partially shown. The coupling plate arrangement forms thesubject matter of copending application 216,228 filed August 10, 1962,and assigned to the same assignee as the present invention.

The negative cathode potential of approximately 23 kv. is applied to thecathode emitter 9 via high voltage lead-in insulator assembly 4. Thecathode 9 uses a low voltage A.C. filament heater and therefore a dualwire cathode lead-in 10 is used.

In operation the 11' mode of the magnetron interation region is lockedto the circular electric mode resonator via the intermediary of thecoupling slots 7 serving to drive the resonator 5. An annular slot modeabsorber 24 juxtapositioned the coupling plate end of the slots 7suppresses the undesired slot mode. The slot mode absorber and anodewall arrangement forms the subject matter of copending application223,499 filed September 13, 1962, and assigned to the same assignee asthe present invention. Output energy from the resonator 5 is extractedvia the coupling plate 13 and transmitted to the load, not shown, viathe intermediary of the circular electric mode output waveguidestructure 2 and output wave permeable window 17.

The tube structure and mode of operation will now be described ingreater detail as it specifically relates to each of the beforementioned features of the present invention.

The novel A.C. cathode heater filament insulating support structurefeature of the present invention is more clearly seen by reference toFIGS. 4, 5, 6 and 7. The A.C. filament support structure ischaracterized by the provision of a plurality of curved refractorylongitudinally directed members being disposed about the periphery ofthe bare A.C. heating filament helix serving to tangentially support thehelix while allowing relative motion between the helix and the supportWithout producing excessive chafing of the insulating members.

More specifically, the cathode emitter ring 9 as of molybdenum isprovided with an enlarged hollow body portion 61 near the outerperiphery of the cathode emitter 9. The hollow interior of the emitter 9forms an annular cathode heater filament nest 62 which in a preferredembodiment has a generally square cross-section. The bare wire heatingfilament 63, as of tungsten wire, is wound into a helix longitudinallydirected of the annular nest 62.

A plurality of annular insulating members or segments 64 as of arefractory insulating material as of, for example, alumina ceramic aredisposed about the periphery of the bare helix 63 at spaced apartpositions, i.e., the corners of the rectangular nest 62, to tangentiallysupport the helix 63. The insulating members 64 are disposed with theirlongitudinal axes directed around the annulus of the nest 62 and serveto prevent the A.C. heating filament 63 from shorting to the conductivewalls of the filament nest 62.

The strong static axial magnetic field, as of 12,000 gauss, in themagnetron interaction region 8 produces a very strong fringing fieldcomponent which threads generally in the axial direction of the tubethrough the filament nest 62. The A.C. heater current supplied to thehelical cathode filament 63 causes the helix to produce relativemovement in accordance with the changing A.C. current, between thefilament 63 and the walls of the nest 62. This movement of the filamentwill hereinafter he referred to as motoring. Previous attempts toutilize a heater wire insulated with a ceramic coating resulted in themotoring causing the insulation to chafe off, through to the filament,in about five minutes of use due to the extremely high operatingtemperature of approximately 1200 C. Once the insulating coating hasworn off the A.C. heater filament, the filament shorts to the walls ofthe cathode heater nest 62 rendering the tube inoperative.

By utilizing the A.C. heater support structure of FIG. 4 the operatinglife of the cathode has been extended from five minutes to an excess of2400 hours.

An alternative A.C. heater filament support structure is shown in FIGS.5 and 6 wherein the insulating members 64 have been replaced by aceramic coated wire 65. This embodiment has the advantage that theceramic coated insulated wire 65 is more economical to procure than thespecially ground insulating rods 64.

The cathode emitter 9 is surrounded by a combined cathode end hat andheat shield assembly 67. More specifically, the heat shield 67 iscomposed of a plurality of annular overlapping plates closely spaced tothe molybdenum cathode body 9. The two cathode end hat annular membersare spaced apart from the thin neck portion of the cathode emitter ring9 via a plurality of detents pressed into the cathode end hat plates.The detents providing a 0.003 spacing between the end hats and thecathode emitter ring 9. The annular heat shield ring segments are allheld together by the provision of a plurality of peripherally spacedaxially directed stainless steel pins 68 passing through the cathodeemitter ring and being provided with localized spacing washers 69serving to provide a separation between the overlapping heat shieldplates.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A cathode heater assembly for use in strong D.C. magnetic fieldsincluding, means for producing a strong magnetic field, a bare helicalA.C. filament cathode heater element having spaced apart turns immersedin the strong magnetic field with the longitudinal axis of the helixbeing directed transversely of the direction of the magnetic field, aplurality of refractory insulating members peripherally spaced apartabout the circumference of the cross-section of the helix and beinglongitudinally directed of the helix for tangentially supporting thebare A.C. heater filament within the cathode structure whereby motoringmovement between the helix and said tangential support is permittedwithout shorting of the A.C. filament to the cathode structure.

2. In a crossed field tube apparatus employing a strong axially directedD.C. magnetic field, an anode structure having a circumferally directedwave supporting structure formed therein, a circumferally directedcathode emitter disposed adjacent said anode wave supporting structureand defining a magnetron interaction region therebetween permeated bythe strong axially directed D.C. magnetic field, a circumferallydirected heater chamber formed in said emitter, a bare filament wirewound into a helical form with axially spaced turns and disposed withinsaid heater chamber, said helix being directed within said heaterchamber about the circumference of said cathode emitter and generallytransversely directed to the direction of the strong D.C. magneticfield, a plurality of refractory insulating members peripherally spacedapart about the circumference of the cross-section of said helix andbeing generally longitudinally directed in the direction of the helixfor tangentially supporting said helical filament within said heaterchamber of said cathode emitter, whereby a motoring motion between thehelix and the tangential support produced by an A.C. current flowtransverse to the direction of the strong magnetic field is permittedwithout shorting said A.C. filament to said cathode structure.

3. The apparatus according to claim 2 Wherein said refractory insulatingmembers are curved rods having a radius of curvature approximately equalto the radius of curvature of said filament helix.

4. The apparatus according to claim 2 wherein said refractory insulatingmembers are curved ceramic coated Wires, said wires having a curvaturesubstantially approximating the radius of curvature of said filamenthelix.

References Cited by the Examiner UNITED STATES PATENTS 2,448,573 9/1948Blazier et al. 313-340 2,482,495 9/1949 Laidig 31339 3,027,481 3 1962Baber ct al. 313-275 FOREIGN PATENTS 787,458 12/ 1957 Great Britain.

10 JOHN W. HUCKERT, Primary Examiner.

JAMES D. KALLAM, Examiner.

1. A CATHODE HEATER ASSEMBLY FOR USE IN STRONG D.C. MAGNETIC FIELDSINCLUDING, MEANS FOR PRODUCING A STRONG MAGNETIC FIELD, A BARE HELICALA.C. FILAMENT CATHODE HEATER ELEMENT HAVING SPACED APART TURNS IMMERSEDIN THE STRONG MAGNETIC FIELD WITH THE LONGITUDINAL AXIS OF THE HELIXBEING DIRECTED TRANSVERSELY OF THE DIRECTION OF THE MAGNETIC FIELD, APLURALITY OF REFRACTORY INSULATING MEMBERS PERIPHERALLY SPACED APARTABOUT THE CIRCUMFERENCE OF THE CROSS-SECTION OF THE HELIX AND BEINGLONGITUDINALLY DIRECTED OF THE HELIX FOR TANGENTIALLY SUPPORTING THEBARE A.C. HEATER FILAMENT WITHIN THE CATHODE STRUCTURE WHEREBY MOTORINGMOVEMENT BETWEEN THE HELIX AND SAID TANGENTIAL SUPPORT IS PERMITTEDWITHOUT SHORTING OF THE A.C. FILAMENT TO THE CATHODE STRUCTURE.